In subsea hydrocarbon production systems, it is common practice to adopt configurations, which are effective in fulfilling several important operational objectives including, but not limited to, flow assurance, operating flexibility and reliability, while also enabling swift and safe fabrication and installation. Such systems also require means of maintaining steady state production, and mitigating critical operational situations such as emergency and planned shut downs, start ups and blockages to flow in production lines that may hinder or cease production. Systems are needed which prevent, intervene, or correct undesirable conditions such as slugging and blockage by hydrates or other solids within production lines. Several existing methodologies and technologies are utilized to maintain flow in production lines, including, but not limited to, the following: thermal insulation, heating, pigging, gas lift, displacement of production fluids, chemicals, coiled tubing intervention into production lines, circulation of hot fluids, de-pressurization of production lines, slug catchers, and well testing.
In subsea hydrocarbon production, components such as risers, flowlines, and jumpers, are frequently used to convey hydrocarbon products from a subsea well to a topsides production platform, vessel or tieback to an onshore plant facility. These components typically are steel pipes or engineered pipes.
Engineered pipes can be fabricated in a number of different configurations. For example, engineered pipes can be bonded or unbonded flexible pipe. In addition, engineered pipes can be composite pipes designed and manufactured with metallic and/or non-metallic materials such as steel, thermoplastic, plastic, elastomers, carbon fiber, polymeric compounds, fiber glass, ceramic compounds and the like. Engineered pipes can be formed as a single layer or multiple layers with each layer designed for a specific function, providing the engineered pipe with special characteristics such as corrosion resistance, lower flexural stiffness, thermal insulation, tensile strength, collapse strength, pressure containment, or abrasion resistance. Engineered pipes are often terminated in devices commonly called end fittings, which acts as a housing to wrap, secure, anchor, and/or seal all the layers and also provide interface connections with other pipe sections, subsea structures, and/or topsides piping.
One example of an engineered pipe comprises an innermost layer (or carcass layer) of unbonded flexible pipe made of corrosion resistant material, designed to resist collapse of the flexible pipe. The carcass layer is surrounded by another polymeric sealant layer or pressure sheath which is extruded around the carcass layer and sealed at flexible pipe end fittings to contain fluid within the bore. Surrounding the polymeric sealant layer is an annulus containing a number of metallic layers designed to impart strength against tensile loading and internal pressure loading. Surrounding these layers is another polymeric sealant layer or external sheath designed to avoid external sea water ingress into inner layers of the flexible pipe, which acts as an outer protective layer.
Another example of engineered pipe comprises a bonded flexible pipe made up of layers of non-metallic materials such as elastomers or polymers, either wrapped or extruded individually and bonded together through the use of adhesives or by applying heat or pressure or a combination of thereof to fuse and bond the layers in to a single wall construction.
A composite pipe is another example of an engineered pipe. This type of pipe may comprise of one layer or multiple layers of non-metallic materials such as thermoplastic, plastic, elastomeric, carbon fiber, polymeric compounds, fiber glass, ceramic compounds and the like. These materials may individually exist in engineered pipe or they may be combined and/or fused together during the manufacturing process to form various composite hybrid materials.
Engineered pipe can also be devised with additional features such as heat tracing. Heat tracing provides active heating by electric cables, which are integrated and bundled into the engineered pipe structure to improve the thermal performance of the system. Engineered pipe can also be devised with fiber optic cable for sensing, process monitoring, and integrity monitoring and data transmission.
A pipe system including a pipe-in-pipe (PIP) apparatus may be utilized to provide enhanced thermal performance in subsea hydrocarbon production components such as flowlines, risers, and jumpers. A typical PIP apparatus includes an inner steel pipe disposed within an outer steel pipe with an annular space between the inner surface of the outer pipe wall and the outer surface of the inner pipe wall. Materials with high insulation properties typically present low (poor) external pressure resistance. The configuration of the PIP apparatus allows the use of these high insulation materials in the annulus between the two pipes because the outer pipe shields the external hydrostatic pressure from the annulus thus protecting the insulation material from damage and degradation. In some applications the PIP has empty and interconnected annular space to allow the flow of fluids through both the inner bore and the annulus space.
However, there are challenges associated with the design of a conventional pipe system using a PIP apparatus. For example, this system requires an increased amount of field fabrication due to the requirement to weld and assemble two pipe strings (inner and outer pipes). In addition, there are other challenges including, but not limited to, large thermal expansion effects and the risk of buckling the materials and connections; low cycle-high stress fatigue loading; corrosion problems with the inner pipe and outer which is exposed to the transported fluids and/or seawater; concerns regarding the integrity of the system when subject to installation loads and the risk of structural failure; the design of bulkheads associated with the PIP apparatus which need to allow both for quick field assembly and also to provide isolation and structural strength; and non-desired electrical continuity between the outer pipe and the inner pipe. Further, an external leak, which will lead to a flooded annulus, can undermine the system performance if not properly contained or limited. It would be desirable to provide a way to address the challenges associated with the conventional PIP apparatus in which both the outer pipe and the inner pipe are made of steel.